It has been known since the late 1980s that the deposition of certain alloys, such as ZnMg, on the surface of a steel strip plays a role in protecting steel. The excellent corrosion resistance of the ZnMg alloy is attributed to the nature of the corrosion products formed on the surface of the strip in an extremely dense layer acting as a barrier film.
Such an alloy deposition is generally only possible by the usual techniques, such as electrolytic deposition, hot dipping, etc., within certain composition limits. Thus, at atmospheric pressure, contamination of the liquid metal bath by the oxygen in the air may occur, which forms oxide mattes on the surface of the bath.
If one wishes to obtain extended thickness and composition ranges, the only possible solution is often the vacuum evaporation of the liquid metal, pure or in alloy form (PVD technique, Pressure Vapor Deposition).
In the context of this technique, it is known to place the substrate in a vacuum enclosure maintained at a low temperature and comprising a crucible of molten metal. The deposition is then achieved on all the walls with a temperature below the temperature of the metal vapor. To increase the deposition yield on the substrate and prevent waste, it is therefore interesting to heat the walls of the enclosure.
In document WO-A-97/47782, a method is described for continuously coating a moving substrate in which the metal vapor is generated by induction heating a crucible containing a bath formed by the coating metal in a vacuum enclosure. The vapor escapes from the crucible via a pipe that brings it towards an outlet orifice, preferably calibrated, so as to form a jet oriented towards the surface of the substrate to be coated. The use of an orifice in the form of a longitudinal slot with a narrow section allows to regulate the vapor mass flow rate, at constant sonic velocity along the slot (sonic throat), which procures the advantage of obtaining a uniform deposition. This technique will henceforth be designated using the acronym “JVD” (Jet Vapor Deposition).
This technology does, however, have several flaws, in particular:                the constant supply of liquid metal involves providing its return to the vat in one or several points;        the liquid metal comprising impurities, there is a concentration of these impurities on the surface of the bath after evaporation, which reduces the flow rate. The uniformity of the bath is necessary to obtain a uniform deposition. It involves bringing cool liquid at a location whilst removing the used liquid at another location. One solution would be skimming the surface or recycling the load, but any mechanical operation is made difficult in a vacuum;        the difficulty of adapting the evaporation slot to a variable bandwidth, which involves concealing means on either side of the slot, and hence the production of vapor sealing under vacuum and at 700° C., which is not easy to do;        the difficulty of concealing the slot when the movement of the strip is interrupted, which would involve the presence of a sealing linear valve over a typical length of 2 meters or more;        the major thermal inertia of the system (at least several minutes);        the heating, done by vacuum induction, requires passing all of the electrical heating power via electrical connectors through the vacuum sealing wall, which does not facilitate the accessibility and maintainability of the facility.        
Furthermore, the state of the art does not provide a satisfactory solution to the need to perform the co-deposition of two distinct metals, involving the mixture of two jets leaving the evaporator. The use of intermediate mixing boxes with baffles did not provide a sufficiently convincing result.
A first way to proceed for depositing an alloy coating on a strip is to first deposit a layer of the first metal, such as zinc, for example by hot dipping, electrolysis or vacuum magnetron spraying, then deposit a layer of a second metal, such as aluminum, for example in a vacuum, and to finally perform a thermal diffusion treatment, for example low temperature annealing, which produces the alloy.
The advantage of this method is that it has a simple design, allowing for a step by step regulation.
A first drawback is, however, multiplying the steps of the method, and therefore its cost. In particular, thermal diffusion treatment consumes a significant amount of energy. For example, if the relative thickness of the coating is 1%, the required energy must be provided to the entire thickness of the finished product, i.e. 100%, which corresponds to several megawatts for an industrial line.
Thus, document WO-A-02/14573 describes the development of a coating from a base zinc plated coating obtained by a conventional hot dipping or electro-galvanizing method, which in turn is then vacuum coated with magnesium. Rapid induction heating allows to postpone the fusion deposition for several seconds and to obtain, after cooling, a favorable microstructural distribution of alloyed phase ZnMg in the entire thickness of the layer.
Document FR 2 843 130 A describes a method for coating a surface with a metal material, according to which:                a first coating of said material is achieved using a metal or metal alloy layer,        a thermal treatment is achieved on the first coating using a rapid heating means so as to bring the surface of said first coating to a temperature below the melting temperature of the metal material, and        a second coating is deposited from a metal or metal alloy.        
The Applicant has also proposed an industrial dual-layer electro-galvanized/ZnMg alloy product obtained by PVD (EP-A-0 756 022), as well as an improvement of the method with an infrared heating system to alloy the magnesium with the zinc in order to minimize the formation of the fragile intermetallic FeZn phase.
A second drawback is that not all types of steel accept this thermal treatment. For example, BH (bake hardening) steels are malleable, deformable, anti-corrosion steels intended for automobiles, which have instabilities that are displaced during curing of the paint, which causes the sheet metal to harden. This product therefore has a difficulty related to hardening that results from its reheating. A direct alloy deposition would therefore allow to overcome these drawbacks.
Another method is therefore to produce metal coating alloys by direct deposition of the alloy without thermal treatment, by rigorously controlling the concentration of both metals in the crucible. For example, if 50% Zn and 50% Mg are placed in the crucible, an alloy of 85% Zn/15% Mg is obtained, given the different evaporation speeds. However, this control involves great difficulties in managing the system, in light of the continuous concentration variation in the crucible. In particular, it is difficult to ensure homogeneity in the crucible, especially if it is not of circular section. For example, POSCO (publication: “Next Generation Automotive Steels at POSCO,” January 2008) proposes a coating obtained by PVD at very high velocity, with a high vapor yield and high energy yield, in particular in the form of an alloy co-deposition from a single evaporation source.
Still another method according to the state of the art consists in using two crucibles, each generating a type of vapor, both generated vapors being oriented by a channel towards a mixing device, from which the alloy is deposited on the strip.
Patent BE 1010720 A3 describes a method for continuously coating a substrate in motion using a metallic alloy in vapor phase, in which the various components of the alloy are evaporated into suitable distinct elements and whereof the different metal vapors obtained are channeled towards the location where the deposition occurs. One of the vapors coming from the metal baths with the components of the metal alloy plays the role of a propellant element relative to the other metal vapors present.
In document WO-A-02/06558, a ZnMg coating is obtained in a vacuum by evaporating from two crucibles, one with zinc and the other with magnesium. Before they are projected on the strip, the vapors are mixed in a throttling device in the form of plates provided with holes or slots, which allows to obtain maximum sonic velocity and vapor flow rate. However, the high speed of the vapors before mixing makes it very difficult to obtain a homogenous mixture by molecular diffusion.
In L. Baptiste et al., “Electromagnetic levitation: A new technology for high rate physical vapour deposition of coatings onto metallic strip”, Surface & Coatings Technology 202 (2007), 1189-1193, a method is proposed based on the levitation technology for conductive materials in high-frequency electromagnetic fields. Through a suitable design of the induction coils, it is possible to obtain high power densities and metals at low vapor pressures such as aluminum, nickel, or copper, as well as their alloys can easily be evaporated. The produced vapor is guided towards the substrate by a specially designed vapor distribution system, which allows to obtain good uniformity of the coating and very wide use of the vapor.
Document U.S. Pat. No. 5,002,837 describes the evaporation deposition of a dual-layer Zn/ZnMg coating with a completely alloyed Zn2Mg or Zn2Mg/Zn11Mg2 phase.
Patent application EP-A-2 048 261, in the name of the Applicant, discloses a vapor generator for depositing a metal coating on a steel strip, comprising a vacuum chamber in the form of an enclosure, provided with means for ensuring a vacuum state therein relative to the outside environment and provided with means allowing the strip to enter and exit, while being essentially sealed relative to the outside environment. This enclosure covers a vapor deposition head, called ejector, configured to create a metallic vapor jet at sonic velocity towards and perpendicular to the surface of the strip. The ejector is in sealed communication via a supply duct with at least one crucible containing a coating metal in liquid form and situated outside the vacuum chamber. The vapor generator comprises means for regulating the flow rate, pressure, and/or speed of the metallic vapor in the ejector. Document EP-A-2 048 261 belongs to the state of the art pursuant to Article 54(3) EPC.
Prior patent application EP-A-1 972 699, in the name of the Applicant, discloses a method and facility for coating a substrate according to which a layer of metallic alloy comprising at least two metal elements is continuously deposited on said substrate, using the vacuum deposition facility comprising a vapor jet coating device, allowing to project on the substrate a vapor comprising the metallic elements in a predetermined relative and constant proportion, the vapor being brought to sonic velocity beforehand. The method is more particularly intended for the deposition of ZnMg coatings.